Thanks for your reply,unfortunately another 1 died this a.m .
The chickens are in a secure pen with no plants,weeds,etc available so they only eat
what we put into their 2 feeders i.e layer pellets,grit,shell and lettuce,cooked potatoes,Cabbage leaves,dandelions,etc....Sometimes we give them old biscuits(dry) and bread.
The remaining hens look fine (at the moment!!)
Have booked them to see a vet so will keep my fingers X'd